The present invention relates to a process for production of a solventless silicone resin, and more specifically, this invention relates to a process for producing a solventless organopolysiloxane resin which exhibits excellent viscosity values at ambient temperatures, and to the resin per se. Organopolysiloxane resins are well known in the art and typically comprise a resin composed of trifunctional and difunctional siloxy units with a silanol content varying from 0.1 to 8.0 percent by weight and optionally, when produced from alkoxysilanes, an alkoxy content varying from 0 to 4 percent by weight. Such silicone resins as set forth above, which are composed of trifunctional and difunctional siloxy units, when dissolved in suitable solvents, have been found useful for forming varnishes and silicone based paints, as well as for resins adapted for molding and encapsulating purposes. The silicone resins are highly desirable for industrial applications since they have better high temperature stability properties when compared to ordinary paints and potting compounds.
Accordingly, a highly successful silicone coating composition typically comprises a suitable solvent or solvent system and, dissolved therein, difunctional and trifunctional siloxy units since the combination of such units imparts to the resin, when cured, desired flexibility as well as the ability to withstand extreme high temperatures and cyclic variations thereof.
The hydrolysis-condensation products of silanes such as methyltrialkoxysilanes, dimethyldialkoxysilanes diphenyldialkoxysilanes, methylphenyldialkoxysilanes and phenyltrialkoxysilanes are of commercial interest because such products possess unique properties among silicone resins. Usually the alkoxy groups in such silanes are methoxy, ethoxy, or occasionally, propoxy, including isopropoxy.
In some instances, it can be difficult to control the hydrolysis and condensation rates of organotrialkoxysilanes. Some of the silanol molecules (formed by hydrolysis of the silanes) which have one or more hydroxyl groups bonded directly to a silicon atom, have the tendency to form the corresponding ether analogs (siloxanes) by an equilibrium reaction that may be regarded as the loss of one molecule of water per two such hydroxyl groups. The chemical reactivity of silanols, especially silanetriols, to form siloxanes is so great that selectivity in product formation is seldom possible and complexity of the reaction mixture often prevents a positive identification of specific products.
The high activity of most silanols makes it desirable to form them in situ. This is usually accomplished by hydrolysis of precursors which contain silicon atoms attached to substituents convertible to hydroxyl groups.